1. Field of the Invention
The present invention relates to a heat developing photosensitive material which permits the formation of an image by dry treatment, and an image forming method using the heat developing photosensitive material.
2. Description of the Related Art
Silver salt photography using a silver halide is a popular recording technique having excellent sensitivity and gradation. However, since treatment after image exposure is performed in a wet manner, this technique has problems with respect to poor workability, simplicity and safety.
On the other hand, research on dry materials which can eliminate dry treatment has been carried out and disclosed in Japanese Patent Publication Nos. 43-4921 and 43-4924. These materials comprise a photosensitive silver halide in a catalytic amount and a non-photosensitive organic silver salt as an image forming material. The organic silver salt is considered to function as an image forming material by the following mechanism:
(1) A latent image is formed on a catalytic amount of photosensitive silver halide by image exposure.
(2) When the photosensitive material is heated, the organic silver salt is reduced to silver by oxidation-reduction reaction of the organic silver salt and a reducing agent in the presence of the latent image as a catalyst to form an image.
A heat developing photosensitive material has the advantage that an image can be formed by dry treatment, not a wet process, and is thus used as a photosensitive material in various industrial fields of image communication, medical care, computer output, and the like. The heat developing photosensitive material contains a silver halide as a photosensitive component, and thus has high photosensitivity and can easily be sensitized within the visible region.
A semiconductor laser which is low-priced, small and lightweight and which has high-efficiency output, as compared with a gas laser, has recently been developed and brought into practical use. A low-priced compact system for high-performance dry image recording can thus be expected by using such a low-priced, small and lightweight laser light source and a heat developing photosensitive material.
In a conventional gelatinous silver halide photosensitive material for wet treatment, a cyanine dye is used for improving the photosensitivity to long-wavelength light, particularly red light. However, since the cyanine dye has an extremely low efficiency of sensitization for the dry treatment heat developing photosensitive material, it is considered to be unsuitable.
The wet treatment silver halide photosensitive material can also decolor a sensitizing dye during wet treatment. However, a conventional heat developing photosensitive material has the problem that decoloring cannot be sufficiently effected in the heating step, thereby causing difficulties in obtaining an image with a low minimum optical density. If the amount of the sensitizing dye added is decreased for reducing the minimum optical density, problems of poor sensitivity and poor resolution can occur.
The conventional heat developing photosensitive material also has the problem that its sensitivity significantly deteriorates if it is stored in an unused state for a long period of time.